Many chemical and physical processes such as catalytic cracking, hydrogenation, oxidation, reduction, drying, polymerization, coating, filtering and the like are carried out in fluidized beds. A fluidized bed briefly, consists of a mass of solid particulate fluidizable material in which the individual particles are neutrally levitated free of each other by fluid drag forces whereby the mass or fluidized bed possesses the characteristics of a liquid. Like a liquid, it will flow or pour freely, there is a hydrostatic head pressure, it seeks a constant level, it will permit the immersion of objects and will support relatively buoyant objects, and in many other properties it acts like a liquid. A fluidized bed is conventionally produced by effecting a flow of a fluid, usually gas, through a porous or perforated plate or membrane underlying the particulate mass, at a sufficient rate to support the individual particles against the force of gravity. Conditions at the minimum fluid flow required to produce the fluid-like, or fluidized condition, i.e., the incipient fluidization point are dependent on many parameters including particle size, particle density, etc. Any increase in the fluid flow beyond the incipient fluidization point causes an expansion of the fluidized bed to accommodate the increased fluid flow until the gas velocity exceeds the free falling velocity of the particles which are then carried out of the apparatus, a condition otherwise known as entrainment.
Fluidized beds possess many desirable attributes, for example, in temperature control, heat transfer, catalytic reactions, and various chemical and physical reactions such as oxidation, reduction, drying, polymerization, coating, diffusion, filtering and the like.
Among the problems associated with fluidized beds, a most basic one is that of bubble formation, frequently resulting in slugging, channeling, spouting, attrition and pneumatic transport. This problem is most common in gas-fluidized systems. Bubbling causes both chemical and mechanical difficulties: for example, in gas-solids reaction gas bubbles may bypass the particles altogether resulting in lowered contacting efficiency while chaotic motion of the bed solids may set up detrimental mechanical stresses tending to deteriorate the vessel and its contents. Many procedures and systems have been proposed to effect improvements, for example, by the use of baffles, gas distribution perforated plates, mechanical vibration and mixing devices, the use of mixed particle sizes, gas plus liquid flow schemes, special flow control valves, etc.
.[.for.]. .Iadd.For .Iaddend.example, U.S. Pat. No. 3,169,835 to Huntley et al disclose that mesh packing throughout the bed breaks up large gaseous bubbles and prevents coalescence of existing bubbles. However, baffle devices do not prevent the initiation of bubble formation.